The broad objective of this proposal is to elucidate the molecular mechanism(s) of genotoxic-induced p53 activation, p53 is activated in response to various cellular stresses, including DNA damage and other stresses such as metabolic change, hypoxia, hyperoxia, heat shock, or oncogene expression. DNA damage leads to phosphorylation and rapid accumulation of p53. However, how DNA damage-mediated phosphorylation induces p53 activation is not entirely clear. We and others have demonstrated that Pinl, a member of peptidyl-prolyl cis/trans isomerases (PPIases), binds p53 in DNA-damage induced phosphorylationdependent manner. Our data indicate that p53 is induced for phosphorylation at the Ser/Thr-Pro motifs that facilitates p53-Pinl interaction and that Pinl is required for timely activation of p53 in DNA damage response. Cells lacking Pinl are impaired in p53-mediated cell cycle checkpoint control and apoptosis in response to DNA damage. Base on these observations, we hypothesize that Pinl-mediated modulation of p53 is a novel regulatory mechanism for p53 regulation in response to DNA damage and that Pinl modulation of p53 is important for chemotherapy of human cancer. This proposal will use an integrated molecular approach to investigate the mechanism(s) by which Pinl modulates p53 and the p53 family member, p63. In addition, we will examine the role of Pinl-mediated modulation of p53 in chemotherapy of breast cancer using an animal model. This study may provide insight into a novel mechanism of activating p53 in response to genotoxic stress, the mechanism of human tumorigenesis, and potentially an approach of identifying a new class of cancer therapeutic targets.